A NAND flash memory has been spread as a memory device storing a large amount of data. In recent years, a memory cell has been scaled down to reduce costs per bit or to increase the amount of data stored in a memory device. In contrast, a new memory device based on an operation principle different from that of a floating-gate flash memory is expected to be put to practical use. For example, a variable resistance memory with two terminals which is typified by a resistive random access memory (ReRAM) promises to be a next-generation memory in terms of a low-voltage operation, a high-speed switching, and ease of scaling-down.
An example of the variable resistance memory with two terminals is a variable resistance memory in which a variable resistance layer is made of metal oxide. In general, an electrical resistance value of metal oxide changes depending on the number of oxygen vacancies in a film. Therefore, a voltage is applied to metal oxide disposed between an upper electrode and a lower electrode to change the distribution of oxygen vacancies in the film. In this way, it is possible to switch the variable resistance layer between a high-resistance state and a low-resistance state.
In recent years, a variable resistance memory has drawn attention which does not include a single metal oxide layer, but includes two or more metal oxide layers and has a function that is not obtained in a single-layer ReRAM. For example, a stacked ReRAM obtained by stacking titanium oxide and aluminum oxide has self-compliance properties and two-dimensional conductivity and has drawn attention.
Since the stacked ReRAM has an asymmetric structure, in many cases, a set voltage (Vset) required to reduce resistance is different from a reset voltage (Vreset) required to increase resistance. For this reason, in some cases, one of the voltages does not satisfy required voltage specifications. Therefore, it is preferable to adjust the balance between the set voltage and the reset voltage in the stacked ReRAM.